Typically, WGM resonators are unique because of their high Q factors and small mode volumes. An important feature of these resonators is the structure of the WGM spectra. Many practical applications require the spectrum of the resonator to be sparse, originating from a single-mode family, and uncontaminated by the presence of other modes.
Some applications of such resonators are particularly burdened with rare and/or inhomogeneous spectra. These applications include coherent cavity ringdown spectroscopy and electro-optic modulation utilizing WGM resonators. For example, in coherent cavity ringdown spectroscopy, narrow absorption features of a substance under study might fall between the resonator modes, thus making the presence of the substance undetectable without tuning of the cavity modes. Additionally, it has been found that reproducing the spectrum in separate runs is difficult if the resonator is not stabilized. In electro-optic modulation, laser radiation that is modulated with a WGM-based electro-optic modulator requires a line-width that is much narrower than the spectral width of a particular mode. In addition, the laser must be locked to the mode of the resonator, or alternatively, the resonator must be locked to the laser, either of which his not always feasible.
Furthermore, in coherent cavity ringdown spectroscopy, the concentration of a particular substance can be measured by placing the substance inside a resonator and injecting an optical pulse into the resonator. The substance inside the resonator then absorbs a wavelength of the injected optical pulse. The concentration of the substance can be determined by measuring the time it takes for the pulse to “ringdown” as it circulates inside of the resonator. A problem with this technique is that the resonant frequency of the resonator must correspond to the absorption frequency of the substance.
A known technique commonly employed to overcome this problem is to inject the optical pulse so that it couples the transverse and longitudinal modes of a non-confocal resonator. These modes overlap and result in a “whitened” spectrum. However, the breadth of this spectrum is limited by the typical nanometer reflectivity width of the dielectric resonator mirrors of the non-confocal resonator.
Accordingly, there exists a need for an optical resonator system and method that provide a “white-light” WGM optical resonator that is capable of resonating across a broad, continuous swath of frequencies, as much as an octave or more, while retaining a high Q factor at all of the resonant frequencies.